The Process of liquid/solid extraction, also known as leaching, involves the transfer of a solute from a solid, generally employed in particulate form, to a liquid solvent which is termed the "extract". In this process, the solid imbibes the solvent which dissolves the solute and thereby extracts it from the solid. The process is very widely used especially in the food industry where it is employed in such diverse tasks as the leaching of cottonseed oil from cottonseed using hexane as the solvent, the leaching of caffeine from coffee beans sing methylene chloride as the solvent, the leaching of sucrose from sugar beets using water as the solvent, and the leaching of minerals from bone, using hydrochloric acid as the solvent. In some leaching processes, as for example in the leaching of minerals from bone, the solvent also serves as a chemical reactant. Thus, the hydrochloric acid reacts with the minerals present in bone to convert them to soluble salts and, accordingly, the leaching process in this instance is actually an extraction/reaction process.
Leaching is a very old process and a great diversity of apparatus has been developed over the years to meet the varying requirements of the process in respect to its widely diverse applications. Leaching processes have been proposed which operate in a batch, semi-batch or continuous mode, and both single-stage and multi-stage contacting techniques are used. Leaching equipment is commercially available for a wide variety of processes, including fixed-bed processes in which solvent is percolated through a stationary bed of solids, moving-bed processes in which the solids are conveyed through the solvent with little or no agitation, and dispersed-solid processes in which the solids are dispersed in the solvent by mechanical agitation.
Among the many critical problems involved in the successful operation of a leaching process are those associated with the fact that the solid material to be leached is often quite heterogeneous in character, and therefore exceedingly difficult to treat in an optimum manner. This problem is especially well illustrated by the difficulties encountered in processes currently used for demineralization of bone.
Cattle bone is one of the important raw materials for the manufacture of photographic gelatin, as well as several grades of food gelatin. Typically, the bone is ground, classified by size, cleaned of fats, grease, blood and bone marrow by hot water washing, and then demineralized in a liquid/solid extraction/reaction process, using dilute acid solution, usually hydrochloric acid, as the solvent/reactant. Removal of the minerals from the bone particles in the demineralization process yields particles of a material known as ossein, similar in size and shape to the original bone particles, which are further processed by means of various well-known steps, typically including a liming step, to produce gelatin. The demineralization is accomplished in large vats filled with the granulated bone to a depth of as much as two meters or more, and flooded with the acid solution so as to completely immerse the bone. The reaction proceeds throughout the settled bed of bone particles until the original acid charge is exhausted. Meanwhile, fresh acid is added at the top of the bed, the liquor containing dissolved acid salts is withdrawn at the bottom of the bed, and carbon dioxide evolves from the top of the bed. After a predetermined time of immersion, usually three to six days depending on the size of the bone particles, the acid liquors are drained. The bed, now composed of particles of ossein, is water washed to remove residual liquor and evacuated as an aqueous slurry.
The granulated cattle bone which serves as the starting material in the gelatin manufacturing process comprises particles of varying size--typically ranging from about 2 to about 30 millimeters in diameter--and of widely varying density--ranging from particles which are hard, non-porous and of high density to particles which are soft, porous and of low density. The problem of treating these heterogeneous particles in a manner which even approximates an optimum for each is clearly a formidably difficult one. The demineralization is essentially a diffusion-controlled process (see P. J. Makarewicz, L. Harasta and S. L. Webb, "Kinetics of Acid Diffusion and Demineralization of Bone". The Journal of Photographic Science, Vol. 28, 177 (1980)) so that larger bone particles require a proportionately longer time of exposure to the acid solution to effect complete demineralization than smaller particles. Similarily, hard non-porous particles require a longer time than soft porous particles. With respect to any given particle the time of contact with the acid solution can be too short--resulting in incomplete demineralization--or too long--resulting in degradation of the ossein and consequent low quality of the gelatin recovered therefrom. Thus, if the process is adjusted to provide an optimum time for large hard, non-porous particles, it will be far too long for small, soft, porous particles, and vice versa.
The actual time at which an individual particle of bone is completely acidulated, i.e., demineralized depends on its size its density, and its location in the bed. Although the bone particles are usually classified before treatment, there are still significant differences in particle diameters within a bone batch. Bone at the top of the bed is completely acidulated first, because it is closer to the fresh acid feed; whereas fresh acid cannot reach the bottom of the bed until the bulk of the bone through which it must percolate is completely acidulated. Dense bone contains more mineral and offers more resistance to acid diffusion within a particle. Therefore, dense bone can be expected to acidulate much more slowly than light bone. In addition, in a large settled bed there are likely to be channels through which acid will move preferentially; and there will be areas where acid flow is restricted by fines, or grease, or partially decomposed sinew. These areas, particularly if they are near the bottom of the bed, will take longer to demineralize than adjacent well-irrigated sectors. Ash residue within the ossein particle and minimally hydrolyzed ossein result from minimal exposure or underexposure to acid.
Ossein exposed to fresh acid is vulnerable to significant degradation by excessive acid hydrolysis. Demineralized particles which, for whatever reason, suffer longer exposure, especially at higher temperature, can expect to be found to have been hydrolyzed more completely. This ossein is more soluble and subject to loss or degradation to undesirably low molecular weight gelatin.
The settled bed, because of its compaction and the slow percolatlon rate of acid liquor, does not promote self-cleaning or flushing. Therefore, sinews, grease and fatty acids tend to remain with the ossein and must be removed by subsequent washing and separation or screening steps. Each of these additional steps results in some loss of ossein and/or extracted gelatin. In addition, the presence of these contaminants unless completely eliminated, potentially lowers the quality of the finished gelatin.
In summary the settled bed process now in use for acidulation of bone produces ossein which is not uniform with respect to degree of demineralization or degree of hydrolysis. Furthermore, the acidulation time necessary to completely acidulate a large bed is excessive for many of the particles therein. Non-uniformity in the acidulation step potentially results in loss of yield and variability in the molecular weight and quality of the gelatin produced.
The problems resulting from the heterogeneous character of cattle bone particles, which are described in some detail hereinabove, are representative of similar problems that occur in a host of other leaching processes in which the starting material is non-homogeneous. To date, a fully satisfactory solution to these problems has not been found, although attempts toward circumventing them to some degree have been proposed in a number of patents including British Pat. No. 1 251 616 and U.S. Pat. Nos. 3,142,667, 3,445,448, and 3,539,549.
It is toward the general objective of providing a novel high-efficiency leaching process, which is applicable to a wide variety of solid materials and which overcomes the aforesaid problems to a very substantial degree, that the present invention is directed. It is a more specific objective of the present invention to provide a novel process for demineralization of bone that facilitates the achievement of enhanced yield and improved quality in the gelatin manufacturing operation.